Jim (Adam Sandler) is a single father with three daughters while Lauren (Drew Barrymore) is also a single mum with 2 sons. They both know a mutual someone who is cancelling their trip to Africa, thus without thinking much they signed up for the trip and get lower rates to Africa with the event activity called blended which the hotel organize for step father, step mother, step daughter, step son and the list goes on.
Like all comedy movie, the formula is overly used. Therefore, you can somewhat predict where the movie is going. Some of the scenes also seems like the formula is either overly used or too stupid to be true. You will know when you watch the movie then.
TL;DR: 2 families trying to be 1 with lots of romance and laughter.